More HP than appears

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  #1  
Old 12-31-03, 09:25 PM
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More HP than appears

Even though I really like that Kabota Diesal lawn tractor with the 54inch cut, its too big when you have to go in between stuff or get around tight obsticles. This is why I like my 8HP model so much because it has 30inch deck and can manover quite well in tight spaces. I have something interesting to tell you cheese that you may not be aware of. When you bring the throtle all the way up you are acustomed to that speed and you think you are getting 8HP right? Well infact you are really not! When I was tinkering with the engine I noticed that I could get the engine riving higher buy manually with my finger pulling up on the throttle idle screw. Its amasing how much speed I am not getting with this B&S L-head engine buy using the mower throttle! My question to you cheese is, how can I get the throttle to pull the throttle higher to get a higher speed which gives me my true 8HP or more?
 
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  #2  
Old 01-01-04, 02:46 AM
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The HP rating of an engine is the product of the speed (RPM) and the torque produced. In an unloaded condition you can get a lot more RPMs out of an engine than in a loaded condition but you aren't producing rated HP because little torque is required to run at the set speed. If you are mowing and hit a patch of heavy grass your engine will slow down and the governor will sense that and advance your throttle to bring the speed back to the set point. Since the engine is now running under a much larger load, torque has to be higher. Usually the rated HP is only produced when the engine is loaded to the point where is can just maintain it's rated RPM at full throttle. The governor will indeed force the engine to full throttle when the load is heavy but won't do so when running unloaded to keep the engine from overspeeding. Bad things can happen to any engine when it runs at an excessive speed. Many larger engines have overspeed trips because if something breaks and the load drops off suddenly while the engine is at full throttle the governor won't be able to reduce the throttle fast enough to keep the engine glued together. Piston rods have been known to knock a large hole in the block after breaking due to an overspeed condition. Something like that could happen to a belt driven mower if a belt should break and you didn't have a working governor.
 

Last edited by jughead; 01-01-04 at 04:17 AM.
  #3  
Old 01-01-04, 10:39 PM
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Right...overrevving is a good way to blow it up. Engine hp ratings are rated in circumstances other than actual application. They often rate them without charging systems, exhaust, cooling fins, air filter, and at higher than normal RPM. Horsepower really doesn't mean much. It is only part of the equation of an engines power. Theoretically, you could have an 8hp engine that wouldn't have enough torque to crank a fishing reel, but you could also have a 1/2 hp engine that could pull a 5000lb load. HP by itself means little. The hp generated by turning the engine at 4000 rpm is almost useless, because the torque is not there. An 8hp engine that is 8 hp at 4000 rpm is a weak engine, but fast. An 8hp engine rated at 1200 rpm will pull a truck, but slow. This is assuming gear & pulley ratios and tire sizes are the same on both machines.
 
  #4  
Old 01-02-04, 02:07 AM
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I believe Kohler is the only company that will give the complete specifications of the hp. Most companies like briggs or tecumseh rate their engines at the max hp they will possibly put out. Kohler lists the max and continuas hp that their engines will put out. Like Cheese said, torque is very important. A 25 hp lawn mower is totally different from a 25 hp tractor. They might have the same hp but the tractor has WAY more torque which is why tractors can pull really heavy loads while lawnmowers are made for mowing lawns. Torque is low speed power. Bulldozers are full of torque.
 
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Old 01-02-04, 02:17 AM
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Horsepower means everything, torque or RPM's by themselves mean little. The HP rating of any engine is simply the RATE at which that engine can do work. Work can be measured in foot-pounds and is called torque when applied to a shaft. Since a shaft can be turning at a particular speed that adds a time factor to the torque and the combination of the two is what is called horsepower. The HP of any engine can be calculated by multiplying the torque (in foot-pounds) by the shaft speed (in RPM's) and dividing the result by 5252. You will get a number that is simply the HP rating of that engine. Now don't get the idea that an engine will produce it's HP rating at all engine speeds or torques. For any given engine there is usually only one combination of speed and torque that will result in max HP. Manufacturers will indeed play games with these numbers to make it look like their engine is a little better. Car companies are famous for doing this. Cheese, an engine turning at 4000 rpm and producing 8 hp is as good as any other and is not a weak engine. Sure, it has a much lower torque than an engine that can make 8 hp at 1200 rpm but it can do the work just as fast. If your load requires 1200 rpm you will have to use a gear box. If you recall as you step down speed with a gear box (or pulley system) you step UP torque. The mathematical combination of torque and RPMs will always yield 8 hp minus the loss in the gear box, of course. In any engine - load combination it's always a challenge to match the characteristics of the engine with the requirements of the load. I'm sure there are combinations out there that are way less than optimum and can make any engine look bad. A gas turbine engine would be an example of an engine that would have difficulty cranking a fishing reel but it does operate at very high speeds. After you get done running the engine through a large gear reduction you will also greatly multiply the torque and still have a reasonable shaft speed left. Some large marine diesel engines turn at extremely slow speeds (about 80 rpm max) but with a very high torque. They don't require a gear box and are hooked up directly to the propeller. If that engine develops, say 30,000 hp, it wouldn't be any better than a large steam turbine engine turning at 10,000 rpm and running through a big gear box that reduces the speed down to 80 rpm's. The moral to this story is any engine - load combination depends on what you want to do with it, how much space you have, how much money you are willing to spend, what kind of fuel you have,....ect. ect. At every turn there are trade offs and compromises. In the end ANY hp is as good as any other no matter how it is generated or how fast or slow it may be turning. Actually getting the engine power to the load at the optimum speed is the trick and could be the reason Cheese was observing a 'weak' engine running at 4000 rpm.
 
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Old 01-02-04, 08:56 AM
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I want to thank all for your replies!!! I have learned so much from this post. And I now no HP means nothing. They just devide the foot in pounds on compression buy the RPM's.
 
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Old 01-02-04, 02:56 PM
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LOL, I was afraid this might get technical when I stated some of the things I did. I was trying to put things in simple perspective. You are right, a 4000rpm 8hp engine is about as good as any 3300rpm 8hp you commonly see, but it is (here's a simplification) "weak" in comparison to the 8hp that is a 1200rpm engine. This is assuming all gears and pulleys ratios are the same. By weak, I mean it does not have the leverage that the engine rated at 1200 rpm does. It's in laymans terms "not as strong"....but it's faster. It can do work faster, but not the same amount of work per revolution. Either engine can be geared to produce the same end result with drive ratio changes.

As far as hp meaning nothing...well, hp by itself, just like torque or rpm by theirselves means nothing. For example: I have a 6hp max motor on my air compressor. It only runs at 1750 if I remember right. I can go get a 6hp motor to replace it with, but if it's rated at 3600rpm, it won't turn the compressor pump...because it hasn't got enough torque. If I change ratio by installing a larger pulley on the motor, then I can make it work. Also, the 16hp engine on my lawnmower won't pull the plow like the 16 hp in the tractor.

Hp does mean a lot terminator...but you get much more understanding of what kind of engine you have when you factor in rpm and torque. A power graph helps even more. This way you can see when the hp comes in the strongest, and same with the torque (at what RPM). The ftlbs of torque is the twisting force on the crankshaft, (in laymans' terms: how much work can it do in one lick) the rpm is how many times per minute the engine makes one full revolution, and the hp is the speed at which the engine can work.

Anyway...just trying to say that hp isn't the sole indicator of an engine's ability or suitability for an application.
 
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Old 01-02-04, 03:32 PM
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Never ask a simple question to an engineer and expect a simple answer. LOL. Cheese is correct, you can always change the motor output speed (and torque) by using a suitable speed modification device (gear box or pulley system) to match the engine to the load. Horsepower by itself is just a number indicating the RATE at which any motor can perform work but says nothing about it's speed or torque.

Blame mother nature for any complexity. If it were up to me, I'd make it simple. When I was in school I often wished for 'simple', but then I remembered, if it were easy any monkey could do it.
 

Last edited by jughead; 01-02-04 at 09:59 PM.
  #9  
Old 01-02-04, 11:10 PM
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Hehe... this subject is one that almost cannot be discussed without getting technical. I think this still hasn't gotten terribly involved like some of the discussions I've been in on the same topic. I have a hard time wording things exactly as they should be when this type of thing comes up, because I almost always try to use a common term (like saying "weak" instead of "less torque"). That's what gets me into trouble. You should have seen me discussing electricity a few months ago...getting very technical...talking about the flow of electrons and electron attraction. I almost blew a mental fuse.
 
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